232 Prof, von Kohell upon Galvanography. 



metal ; 2d, it should be insoluble in the precipitation fluid ; 3d, 

 its surface should be dull when dry ; and 4th, it should to a cer- 

 tain extent possess a conducting power. It need scarcely be re- 

 marked that a soluble paint, or one that does not adhere entirely 

 to the surface on which it is laid, is quite inapplicable for such a 

 purpose ; for, in the latter case, copper insinuates itself beneath it, 

 and instead of a sunk surface, there is produced a flat one, or one 

 that is neither one thing nor another : in the former case, we 

 obtain no picture at all. It is a matter of not less importance 

 that the paint should dry with a dull surface. This is equally 

 indispensable, both for the production of good plates, and for ob- 

 taining satisfactory impressions. It has been already remarked, 

 that the galvanic deposit of copper consists of infinitely minute 

 crystals, which, having once begun to form, attach themselves, 

 as does every other substance under the influence of crystalliza- 

 tion, far more readily to rough than to smooth surfaces, supposing 

 the latter not to be possessed of conducting power. 



If a brilliant varnish is employed with the color, copper it is 

 true may be thrown down upon it, not however without diffi- 

 culty, and we soon see that the little nodules deposited in the first 

 instance, have a far greater tendency to increase upwards, than to 

 extend themselves in a lateral direction. This leads to the form- 

 ation of spherical aggregations, which, with a long continuance 

 of the process, to a certain extent it is true, unite, but between 

 which little holes are frequently left upon the surface of the pic- 

 ture, which do not fill up, and which, when the plate attains a 

 certain thickness, become arched over, as it were. This natu- 

 rally delays considerably the complete coating over of the pic- 

 ture, and as the metallic portions conduct with far greater readi- 

 ness than the vacant spaces, frequent recourse to the file becomes 

 requisite, thus rendering parts already covered liable to be again 

 detached. These nodules moreover do not present a truly uni- 

 form surface below, but have always a tendency to radiation 

 from a central point, so much so indeed, that it is sometimes 

 visible in impressions taken from such a plate. 



If by any means these vacant spaces have a conducting power 

 imparted to them, a galvanic plate may, it is true, be formed 

 easily enough ; one however that does not answer well for taking 

 impressions from ; for, on washing the plate, the whole of the ink 

 is removed from these former hollows. No copper plate is fit for 



